The present invention relates to an air-fuel ratio feedback control method for internal combustion engines, and more particularly to an air-fuel ratio feedback control method for an internal combustion engine provided with a plurality of fuel injection valves operating in respective different operating regions of the engine.
An air-fuel ratio feedback control method for internal combustion engines has been proposed by the present applicant, e.g. in Japanese Provisional Patent Publication (Kokai) No. 62-157252, in which during operation of the engine in an air-fuel ratio feedback control region, the air-fuel ratio of an air-fuel mixture supplied to the engine is controlled by the use of a coefficient which varies with change in the output of an exhaust gas ingredient concentration sensor arranged in the exhaust system of the engine.
This proposed control method is characterized by determining in which of a feedback control region and an operating region other than the feedback control region the engine is operating; when the engine is operating in the feedback control region, determining in which of an idling region, an operating region other than the idling region, and a predetermined acceleration operating region and engine is operating; when the engine is operation in the idling region, the operating region other than the idling region, or the predetermined acceleration operating region, an average value of values of the coefficient obtained in each region is calculated and stored for use in each region; when the engine has shifted to one of these operating regions within the feedback control region, the average value stored for the one region to which the engine has shifted is used as an initial value of the coefficient to thereby start the air-fuel ratio feedback control. Thus, the coefficient can be set to a proper initial value at the start of the feedback control, whereby the accuracy of the feedback control is improved.
However, the proposed control method has the drawback that when the method is applied to an internal combustion engine of a type in which fuel is supplied thereto through a plurality of fuel injection valves arranged in an intake pipe at respective different locations and operating in respective different operating regions of the engine, satisfactory accuracy of the feedback control cannot be secured.
More specifically, this type of engine is so constructed that a plurality of fuel injection valves operate in respective different operating regions. Therefore, the fuel injection valves have different injection flow rate characteristics. Further, the fuel injection valves are provided in the intake pipe at different locations, e.g., at a location upstream of a throttle valve and a location downstream of same. Therefore, if the above-described conventional control method is applied to this type of engine, when the engine has shifted from one operating region in which one fuel injection valve is to operate to another operating region in which another fuel injection valve is to operate, the air-fuel ratio is varied due to difference in the locations from which fuel is injected. For example, when the engine has shifted from an operating region of a fuel injection valve located downstream of the throttle valve to an operating region of a fuel injection valve located upstream of same, fuel injected from the fuel injection valve located upstream of the throttle valve, which is farther from cylinders, does not immediately reach the cylinders, and further, part of the injected fuel adheres to the throttle valve and the interior wall of the intake pipe, so that the air-fuel ratio is leaned temporarily immediately after the transition of operating region. Therefore, the responsiveness of the engine to transition from an operating region of one fuel injection valve to an operating region of another fuel injection valve is not satisfactory, resulting in the degraded accuracy of the feedback control.